Benzotriazole, including mercaptobenzothiazole and tolyltriazole, are known copper corrosion inhibitors, as documented for example in U.S. Pat. No. 4,675,158 which discloses tolyltriazole/mercaptobenzothiazole compositions as corrosion inhibitors. Similarly, U.S. Pat. No. 4,744,950 discloses the use of lower (3-6 carbon) alkylbenzotriazoles as corrosion inhibitors. U.S. Pat. No. 4,338,209 identifies metal corrosion inhibitors containing one or more of mercaptobenzothiazole, tolyltriazole and benzotriazole. Additional triazole corrosion inhibitor patents include U.S. Pat. Nos. 4,406,811, 4,363,913, 2,861,078, and, possibly most notably, U.S. Pat. No. 5,217,686, the latter of which is directed to a composition containing a tolyltriazole or a derivative thereof in admixture with a C.sub.3 -C.sub.12 alkoxybenzotriazole. U.S. Pat. Nos. 5,219,523 and 5,236,626 issued on continuation and divisional applications, respectively, of the application which eventuated U.S. Pat. No. 5,217,686. Related prior art includes U.S. Pat. No. 4,873,139, which discloses the use of 1-Phenyl-1H-Tetrazole-5-Thiol to prepare corrosion resistant silver and copper surfaces. Chemical Abstract CA 95(6) :47253 (1979) similarly discloses the use of 1-Phenyl-5-Mercaptotetrazole to inhibit the corrosion of carbon steel in nitric acid solutions.
In general, benzotriazole and its derivatives of these types and their performance in industrial water systems are judged by their passivation and persistency characteristics. "Passivation" refers to the formation of a film which lowers the corrosion rate of the metallic surface being treated, usually by continuously or intermittently charging a dose of the film forming material directly into the water of the system to be treated. "Passivation rate" thus refers to the time required to form a protective film on a metallic surface, and "persistency" refers to the length of time a protective film is present on a metallic surface when a corrosion inhibitor is not present in an aqueous system which is in contact with the protected metallic surface. Improved film persistence is acknowledged as one of the most important criteria for film-forming corrosion inhibitors of this type, in view of the economic and ecologic advantages of the commensurate low dose or charge required for materials that can attain it. Passivation rate is also important for the same reasons. In other words, those materials whose corrosion inhibiting films are the most valuable of all are those which both form quickly, thus minimizing the presence of the material in the effluent, and which persist for greatest length of time, likewise minimizing the need to charge the material to the system. The present compositions provide such an improvement in that they give enhanced passivation at improved passivation rates and also improved film persistence over benzotriazole and its derivatives similarly employed in the prior art.